An increasing number of portable electronic products are available today which operate on a battery source within the device. These products include such things as cellular telephones, portable radios, pagers and voice recorders which are conveniently mobile and operate using rechargeable batteries. Many different battery chemistries have been used for many years which meet the need for recharging capability. Probably the most popular chemistries include nickel cadmium and nickel metal hydride. A relatively new chemistry, however, generally referred to as lithium ion, enables a cell to be recharged while offering many advantages over other types of rechargeable cells. These benefits primarily are directed to low weight and overall size with a high energy density. One unique factor to be considered when using, a lithium ion cell is its charging scheme. A lithium ion cell is not charged in the same manner as cells utilizing a nickel chemistry.
Nickel-cadmium and nickel metal hydride cells are typically charged using a rapid charge by applying a constant current until a certain event occurs. This event may be coupled to the cell reaching a predetermined high voltage, decreasing to a predetermined low voltage or an increase in the cell's temperature. This is in contrast with the lithium ion cell which requires a two step charging process to achieve optimum performance. The first step of this process provides that the battery charger apply a constant current level while the cell's voltage remains below a predetermined threshold. Once the voltage increases to that threshold, the second step insures the battery charger is held at the threshold voltage allowing the current to decrease. Once the current decreases sufficiently to a desired level, the lithium ion cell is fully recharged.
This two step process presents a problem when considering charging lithium ion cells in a charger designed for nickel systems. Generally, nickel system chargers apply only a constant current which allows the voltage of the cells to rise unimpeded. The voltage may rise to any level provided the battery does not become too hot, i.e. increase to a undesired and dangerous level. Once the nickel system battery becomes hot, the charger detects this state and switches from the rapid high current charge to a value of approximately 5-10% that of the rapid current value. This lower current mode is generally referred to as a trickle current or trickle charge.
Hence, the charging scheme offered by current nickel system chargers will not properly charge a lithium ion cell. Should a lithium ion cell be placed or forced in to the nickel system charger the result could be potentially dangerous since the lithium ion cell could quickly overheat. Therefore, the need exists for a battery charging circuit or system which can be retrofitted to the control circuitry of an existing lithium ion cell allowing the cell to safely use a nickel system charger.
In addition to supplying a retrofitable circuit allowing lithium ion batteries to be recharged using nickel system chargers, a complete battery system would also be useful which would supply additional systems to insure safety when recharging a lithium ion cell in this way.
Still yet another problem associated with utilizing a lithium ion cell with a nickel system charger occurs when circuitry, which is part of the battery system, has disconnected the lithium ion cell from the charging terminals used to charge the cell. The event may occur to due high current or voltage conditions and, due to no operating voltage being available, will leave many of the control and safety systems associated with the battery without power. Thus, no power is present at the charging terminals of the battery.
There are two ways in which a nickel based battery charging system typically detects when to charge an attached battery. These include detection of a thermistor and/or a voltage on the battery. Many charging systems in current use apply a very brief pulse when a data line is detected upon initial connection of the battery to the charging network. Although this pulse is enough to enable or wake up the battery controller, this remains ineffective. Therefore, one method which has been used to provide a start-up or initiation voltage to the internal system to restore voltage to the charging contacts is so called the "double insertion method." This involves actually disconnecting the battery from the charging system once inserted and then reconnecting it at second time. The battery must be reconnected twice since by the time the battery controller is actuated, and the battery controller allows the cell voltage to be applied to the charging contacts, the charger system has already made a determination that no battery is connected. Hence, the battery remains connected to the charging system without any annunciation by the charging system, the consumer understands this to mean that the battery is dead when in fact it is merely disabled. However, once the battery is disconnected and then reconnected, the charger recognizes a voltage potential on the charging terminals since the battery controller was restarted with the first voltage or pulse.
The "double insertion method" is burdensome and confusing requiring connection and reconnection of a disabled battery before recharging can begin. Therefore, the need exists to provide a device and method which allows a charging system to enable a battery controller within a disabled lithium ion battery without the inconvenience of having to connect, disconnect and reconnect the battery to the charging system.